The present invention relates to sheet materials used in forming envelopes for protecting electronic components such as metal oxide semiconductors from potentially damaging electrostatic charges before they are installed in an electronic circuit.
According to a Jet Propulsion Laboratory Report, as many as 60% of some manufacturers' metal oxide semiconductors have been damaged by electrostatic discharge during handling of the semiconductor from the time it is manufactured to the time it is installed in an electronic circuit. Thus much effort has gone into developing means for protecting such electronic components from electrostatic discharges prior to their installation.
Generally two different means for protecting such electronic components are presently in use. Either the leads of each component are shorted together via a metal or conductive plastic polymeric sheet member, or the entire component is enclosed in a protective envelope.
While shorting of the leads with a metal or conductive plastic member can be effective for many applications, removal of the device can be inconvenient during quality control inspection and installation of the component, and materials may be sloughed from the member which can contaminate some components for particular applications.
Prior art protective envelopes have also presented problems. One type of protective envelope has been made from a transparent, sheet material comprising an electrically non-conductive polymeric sheet both surfaces of which material have been made conductive (i.e. having a surface resistivity in the range of about 10.sup.8 to 10.sup.14 ohms per square) by either a special treatment on the polymeric sheet, a coating of an antistatic material on both surfaces of the polymeric sheet or by an antistatic material disposed throughout the polymeric sheet which antistatic material does not significantly increase the volume conductivity of the polymeric sheet. Such resistivity on the surfaces of the envelope, however, does not always allow the envelope to assume the electrostatic potential of a person opening the envelope before the component within the envelope is removed. This can result in a damaging electrostatic charge being transferred from the person to the component after the envelope is opened. Also, since charges on a person handling the envelope are not rapidly dissipated by the outer surface, they can be capacitively coupled through the envelope and damage a component therein.
Other sheet materials for forming protective envelopes have included volume conductive carbon loaded polymeric sheets or metal foils. Envelopes made from these sheet materials are opaque and therefore do not afford visual inspection and identification of an electronic component within. Contact between the component and the inner surface of the envelope will cause metal or carbon filled scrapings which can contaminate the component for some applications. Also such volume conductive sheet materials can conduct electrostatic charges to the component within the envelope. While these latter two problems have been controlled by using a polymeric bag over the component within the envelope, this presents still further problems in that the inner surface of the plastic bag will not dissipate charges on the component caused during manufacture, and further damaging charges may be developed during handling of the envelope by relative movement between the component and the inner surface of the polymeric bag.